Web tracking device



1 1963 J. w. FUNK ETAL 3,368,726

3 WEB TRACKING DEVICE Filed Feb. 26, 1965 4 Sheets-Sheet l /N VENTORS. JOHN W FUN/f. SYLVESTER E PELUWSKI. RICHARD 6. LAPHAM. CHR/5TOP/"IER I). ANDREWS.

FRANK W. BARNES.

AGENI Feb. 13, 1968 J. w. FUNK ETAL 3,368,726

WEB TRACKING DEVICE Filed Feb. 26, 1965 4 Sheets-Shet 2 Fig.2..

LEFT RIGHT SENSING "SENSING IINIT uNIT A B I; A I3. W w w l5 r' "I v I I9 LEFT SOLENOID 25 I 2 I ROLLER l7\: SOLENOID I ASSEMBLY I LOGIC UNIT DRIVER I I UNIT I RIGHT SOLENOID I 22 I IROLLER [25 I 414M I MM ASSEMBLY MACHIIIIAENDCONTROL wCWT S 2w INDICATOR LOGIC I Fig. 9.

INPUT Feb. 13, 1968 J W. FUNK EITAL r, 3,368,726

WEB'TRACKING DEVICE Filed Feb. 26, 1965 4 Sheets-Sheet 3 Fig. 7.

MACH NNDCONTR0L ALARM INDICATOR LOGIC MEANS 1 T0 BOTH Y3 7 SOLENOID A-B-c "k DRIVERS J. W. FUNK ETAL WEB TRACKING DEVICE Filed Feb. 26, 1965 4 Sheets-Sheet 4 v 0? 2 2 i2 5 Q E 5 2 s mgwm 9258 l V V :5 5 m =2 2 q is 22 5- m 2 22 2 0? wzww E g 1 2. A s :5; 2w x 2m 2 mm 55 E c6252 22 E2 0? 2Q 22 3 a: o g 2 $25 51 me a c: E 22 5 Z 5 2 2 United States Patent M 3,368,726 WEB TRACKING DEVICE John W. Funk, Dearborn, Sylvester F. Pelowski, Warren,

Richard G. Lapham, Livonia, and Christopher D. Andrews, Nankin Township, Mich., assignors to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Filed Feb. 26, 1965, Ser. No. 435,572 8 Claims. (Cl. 22617) ABSTRACT OF THE DISCLOSURE An apparatus for detecting, correcting and indicating different normal and abnormal tracking conditions of a moving web. Photoelectric cells having binary outputs are used to monitor the position of the moving web. Logic circuitry translates the various combinations of the binary outputs into signals representing different web conditions. These signals selectively control electromagnetically adjustable rollers which engage the web to laterally shift its position. The logic circuitry also drives audible or visual indicating and alarm devices for signifying the particular web conditions detected.

This invention relates to a web tracking device and, more specifically, to a ribbon tracking device which is capable of sensing, indicating and/or correcting a plurality of ribbon conditions as the ribbon passes through a processing station.

In modern high-speed printers of the types which are frequently employed in conjunction with electronic computers, a ribbon 18 inches wide may be advanced at approximately two or three inches per second with printing occurring in excess of 700" lines per minute. A loop of about two feet of ribbon often must be allowed for feeding over the printing platen. These printers, although called line printers, actually print each line in a series of single characters. Great drag is placed on various sections of the ribbon by the print hammers, causing the ribbon to tend to skew. This compounds the normal tendency of such a wide ribbon to track in one direction or the other due to the grain of the ribbon and the man ner of rolling during its manufacturing process.

The ribbon sometimes tracks inwardly on both sides with wrinkling in the intermediate area. The tensions on the ribbon, particularly if increased by skewing or creasing, may cause the ribbon to tear.

Any tendency of the ribbon to skew or wrinkle must be corrected immediately. In a few seconds of skewing or wrinkling, part of the print hammers may be missing the ribbon, or the ribbon may be binding against the sides of the ribbon guide, and jam the ribbon feeding mechanism. If the ribbon starts to tear, an indication must be given immediately to the operator to avoid damage to the machine and loss of the printed-out information.

Due to the high cost of computer installations, it is of course imperative that they be operated with as little down time as is possible. It will be appreciated that a ribbon tracking malfunction will seriously hammer, if not stop, a computer operation resulting in increased cost and inconvenience to the operator.

Various devices have been used heretofore to detect and correct for the transverse movement of a continuous web of indeterminate length as the web is being transported from one reel to another. Included in these are means for laterally moving the takeup or supply reels when skewing of the web is detected. This method requires a substantial amount of hardware varying in size depending upon the weight of the web being transported. Other systems have utilized pinch rollers to grasp the edges of the web which imparts a force opposite to the 3,368,726 Patented Feb. 13, 1968 ICC direction of ribbon displacement. Many of these devices have operated in a timed relationship with the paper advance in a printer while others have periodically corrected for the lateral displacement.

Various means have been utilized to detect the lateral movement of the web including feelers and a pair of electric eyes. These devices were able to detect movement to the right or left of the home position by the web but were not concerned with, nor were they able to detect, or discriminate against other conditions of the ribbon which were sensed as it moved longitudinally from a supply reel to the takeup reel.

For example, consider the situation where the pair of electric eye units is placed adjacent the right edge of the web, with one unit normally covered and the other normally not covered by the web. Such units are incapable of detecting the difference between a left skewed web, a folded web or no Web at all. Similarly, they are incapable of detecting the difference between a right skewed web, a web too wide, a torn web, or when the lights of the electric eye units are extinguished. In the first case shown, the correction unit would take action to shift the web to the right for all three situations, while in the second case the corrective action would be to shift the web to the left. Any action taken by the corrective device other than to correct for right or left skewing would result in improper tracking and possible faulty print-out.

It is, therefore, an object of this invention to provide a simple and comparatively inexpensive electro-mechanical attachment which can recognize and correct for a plurality of abnormal conditions in the advancement of a web of indeterminate length through a processing station.

It is an additional object of this invention to signal automatically when an advancing web has an abnormal condition.

It is another object of this invention to monitor web travel past a processing station and to report automatically any malfunction.

It is a still further object of this invention to compensate automatically for displacement of an advancing web, returning the web to its home position.

It is an additional object of this invention to improve existing ribbon tracking devices.

In accomplishing these and other objectives, applicants invention comprises a pair of solenoid-controlled assemblies located for rolling cooperation with the longitudinal edges of a wide ink ribbon, a first photo-electric sensing means having at least two photo-electric elements located for cooperation with and in a line substantially perpendicular to one edge of said ribbon, each of said elements having a binary output, a second photo-electric sensing means having at leastone photo-electric element located for cooperation with the other edge of the ribbon, said one photo-electric element normally being blocked by said ribbon and having a binary output, logic means for gating said binary outputs from said photo-electric sensing means, solenoid driver means for energizing and de-energizing said solenoid controlled roller assemblies, either individually or in combination in response to selected signals from said logic means, and means controlled by said logic means for routing condition representative signals from said logic means to machine control and indicator means.

The novel features of the invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description when read in connection with the accompanying drawings in which:

FIG. 1 is an isometric view of a ribbon feed device incorporating the photo-electric sensing units and the solenoid roller assemblies of the invention.

FIG. 2 shows the invention in block diagram form.

FIG. 3 is a front elevation view showing the photoelectric sensing units and light sources with relation to a ribbon passing through the tracking device.

FIG. 4 is an elevation view showing one solenoid roller assembly in the de-energized position.

FIG. 5 is an isometric view of the linkage incorporated in the solenoid roller assemblies.

FIG. 6 is a top plan view showing the relative positions of the photo-electric sensing units and the rollers.

FIG. 7 is a schematic diagram representing the basic logic used in the invention.

FIG. 8 is a schematic diagram of another embodiment of the invention.

FIG. 9 is a schematic representation of one type of amplifier driver which may be used in the solenoid driver unit.

FIG. 1 shows a supply roller 27 having a ribbon 29 wound thereon, which is fed through the left and right photo-electric sensing units 13 and 15, respectively, and through the left and right solenoid roller assemblies 23 and 2-5, respectively, to a take-up roller 31. Left and right are taken arbitrarily for convenience herein as shown in FIG. 1. It is to be understood that the arrangement can be reversed and that applicants invention can be used on an advancing web.

As shown in FIG. 2, the control circuits, represented by the box 11, receive a single input A from the left photo-electric sensing unit 13 and a pair of inputs B and C from the right photo-electric sensing unit 15. Inputs A, B and C, which are responsive to conditions of the web, as hereinafter explained in detail, are utilized in the logic unit 17 to signal the sensed conditions to the solenoid driver unit 19 and to the machine control and indicator logic 21. In response to signals from logic unit 17, the solenoid driver units energize or tie-energize the left sole noid roller assembly 23 and the right solenoid roller assembly 25, either individually or in combination.

The machine control and indicator logic 21 is responsive to all signals from logic unit 17. Some of these signals, such as one indicating a normal tracking condition, may simply indicate while others, such as a torn ribbon or no ribbon, will initiate an alarm means 22 to alert the operator to a malfunction.

The left-photo-electric sensing unit 13, as shown in FIG. 3 has a light source 33 and a single photo-electric element 35 cooperatively positioned in relation to said light source 33 and connected by input line A to the logic unit 17 as shown in FIG. 2. The right photo-electric sensing unit 15 has a light source 37 and a pair of photo-electric elements 39 and 4-1, respectively, cooperatively positioned in relation to said light source 37 and connected by input lines B and C to logic unit 17 as shown in FIG. 2.

In the preferred embodiment, the photo-electric elements 35, 39 and 41 lie along a common axis which is perpendicular to the longitudinal movement of the ribbon 29 as it is moved from the supply roller 27 to the takeup roller 31 or vice versa.

When ribbon 29 is in its normal home position, photoelectric elements 35 and 39 are blocked by said ribbon 29 from the light sources 33 and 37 respectively, and thereby have a binary output of one. Under these normal conditions, photoelectric element 41 is not blocked by the ribbon 29 and thereby has a binary output of zero.

Also, as shown in FIG. 3, the photo-electric elements 39 and 41 are mounted on an L-shaped member 43 which extends above and overhangs the ribbon 29. Likewise, photo-electric element 35 is mounted on an L-shaped member 45 which also extends above and overhangs the ribbon 29. Both L-shaped members 43 and 45 are mounted on a frame 47.

Due to the positioning of the photo-electric sensing elements 35, 39 and 41, any appreciable transverse movement of the ribbon 29 or the edges thereof, caused either by skewing or other abnormal ribbon conditions, will result in the unblocking or blocking of one or more of the photoelectric sensing elements, with respect to the light sources 33 and 37. This results in a change in the binary output which is sent to the logic means 17 or FIG. 2 and will be discussed hereinafter.

Construction of the solenoid roller assemblies 23 and 25 is shown in FIG. 4. Each roller assembly comprises a first arm 53 which is pivotally mounted to the frame 47 at pivot points 49 and 51, as shown in FIG. 6, for the left and right roller assemblies, respectively. Arm 53 has a lower roller 55 rotatably mounted thereon. An actuating arm 57 is pivotally mounted to said first arm 53. The lower end of actuating arm 57 is connected by linkage 59 to a solenoid 61, and upper roller 63 is rotatably mounted on the upper end of actuating arm 57.

Biasing means 65 interconnects the first arm 53 and the actuating arm 57 so that the upper roller 63 is in rolling contact with the lower roller 55 when the solenoid 61 is de-energized. The linkage 59 is drawn toward the solenoid 61, upon energization, causing actuating arm 57 to rotate clockwise about its pivot point on first arm 53 again biasing means 65. This clockwise rotation, in the arrangement as shown in FIG. 4, causes the upper roller 63 to be no longer in rolling contact with the ribbon and through it with lower roller 55. Upon de-energization of the solenoid 61, the actuating arm 57 rotates counterclockwise under the influence of biasing means 65 until the upper roller 63 is again in rolling contact with the lower roller 55 through the ribbon. Both the left solenoid roller assembly 23 and the right solenoid roller assembly 25 are identical in construction and may be energized either independently or in combination by the solenoid driver unit 19 in response to signals from the logic unit 17 as shown in FIG. 2.

The solenoid roller assemblies 23 and 25 are pivotally mounted to the frame 47 to allow said roller assemblies to pivot in the direction of movement of said ribbon 29. The pivoting capability is necessary only when applicants invention is being utilized in a web moving device wherein the direction of travel of the web is reversible. It will be apparent to one skilled in the art that when the web has only one direction of travel, the roller assemblies can be permanently mounted without altering the capability of said roller assemblies.

The details of linkage 59 are shown in FIG. 5 wherein the solenoid plunger '67 is interconnected to a roller connection member 69 by a stainless steel intermediate member 71. The non-magnetic stainless steel intermediate member 71 prevents flux leakage between the solenoid 61 and the roller connection member 69, thereby allowing rapid closure of the upper roller. 63 against the lower roller 55 under the influence of the biasing means 65. The roller connection member 69 may be joined to the arm 57 by any convenient means such as a slot and detent (not shown).

FIG. 6 also shows the ribbon 29 in its normal home position in which photo-electric elements 35 and 39 are blocked by said ribbon 29 from the light sources while photo-electric element 41 is exposed to a light source. In this condition, both left and right solenoid assemblies 23 and 25 are in rolling contact with the ribbon 29 and, due to the motion of ribbon 29, they are skewed in the direction of motion about pivot points 49 and 51 respectively. In this position, the roller assemblies 23 and 25 exert an equal pulling force having vectors which are perpendicular to the direction of travel of the ribbon 29 and cause lateral tensioning of the ribbon. This is true regardless of whether the ribbon is moving from the supply 27 to the take-up reel 31 or vice versa.

Table 1 below shows the truth table which was developed to represent various conditions sensed by the photoelectric sensing means 13 and 15.

TABLE 1 DserlrxsorI ar A B C LightEl Left Right Right Norm. Left Right Norm. Ribbon Condition Left Tracker Right Tracker Remarks Exposed Sensor Sensor Exp. ollers Rollers Sensor I E I 1 1 1 N? lights or ribbon Open Open No rollers pulling. W1 e. I I El 1 l 0 Correct or Normal- Closed- Olosed Both rollers pulling. I E] I l 0 1 Tom ribbon do do.. Do. I U [I 1 0 0 Ribbon tracking Open do Right rollers left. pulling. E! I I 0 1 1 Ribbon tracking Closed- Open Left rollers right. pulling. II] I E] 0 l 0 Ribbon folded in do.. Closed- Both rollers center. pulling. E! E! U 0 0 0 No ribbon Open Open N o rollers pulling.

For example, if the ribbon being utilized is too Wide, or the lights 33 and 37 are extinguished, or, more pro-perly, obscured or covered, all three photo-electric sensors 35, 39 and 41 would be dark andan output of 111 or a binary 7 would be sensed in the logic section 17 of FIG. 2. As shown in FIG. 7, which is one mechanization of Table 1, this would be equivalent to the presence of the signals A, B and C as input to AND gate 73. The output of AND gate 73 is sent to machine control and indicator logic 21 and also sent through OR gate 75 to solenoid driver unit 19 which will energize both left and right solenoid roller assemblies 23 and 25, respectively, resulting in neither roller assembly pulling on the ribbon. The machine control and indicator logic 21 immediately signal alarm means 22 of a tape tracking malfunction. This function may be accomplished by utilizing any of several well-known switching devices such as relays or flip-flops. The alarm generated may be visual or audible, depending upon the system requirements.

Logic gate 77 has an output when no signal is received a on any of A, B or C input lines and is indicative of no ribbon being present. This signal is also sent to machine control and indicator logic 21 and routed through OR gate 75 to solenoid driver unit 17 which in turn energizes both the left and right solenoid roller assemblies 23 and 25, respectively. The machine control and indicator logic 21 will also signal the alarm means 22 of this malfunction.

Logic gate 79 has an output only when signal A is present as an input and signal inputs B and C are simultaneously absent. This is indicative that the ribbon is tracking left and a signal is sent through the appropriate circuitry to machine control and indicator logic 21 and to energize the left solenoid roller assembly 23, releasing the pressure of the left pinch rollers. This causes the ribbon to skew to the right, back to the home position, due to the force exerted on the ribbon by the right roller assembly 25. Upon return to the home position, the binary outputs of the photo-electric sensors 35, 39 and 41 change to a normal indication. This causes de-energization of the solenoid which allows the actuating arm 57 to pivot under the influence of biasing means 65 so that upper roller 63 is in rolling contact with the ribbon 29 and the lower roller 55.

Logic gate 81 has an output only when signals B and C are present as inputs and signal input A is simultaneously absent. This indicates that the ribbon is tracking right and a signal is sent to machine control and indicator logic 21 and to energize the right solenoid roller assem bly 25. This causes the right roller to disengage and the ribbon to skew slightly left under the force exerted thereon by the left roller assembly 23. When the ribbon 29 has returned to its normal condition, the photo-electric sensors 35, 39 and 41 have outputs which allow the deenergization of the right roller assembly 25, allowing it to engage again the right longitudinal edge of ribbon 29. Either a right skewed or a left skewed condition could be indicated on. a machine control and indicator logic 21 in combination with alarm means 22 if the parameters of the system require it.

Logic gate 83 has an output only when signals are simultaneously present on input lines A and B and absent on line C. This indicates that the ribbon is in the home position and tracking properly. In this case, the output signal is sent only to machine control and indicator logic 21.

Logic gate '85 has an output only when signals are simultaneously present on input lines A and C and absent one line B. This combination is indicative of a torn ribbon, and is routed to machine control and indicator logic 21 for application of 'a signal to alarm means 22.

Logic gate 87 has an output only when there is a signal present on input line B simultaneously with the absence of signals on lines A and C, which indicates that the ribbon is folded at the center. This output is also routed to machine control and indicator logic 21 where it is sent to alarm means 22.

Au embodiment without the machine control and indicator logic is shown in FIG. 8, wherein photo-electric sensors 35, 39 and 41 have their respective outputs fed into signal translating means 89, 91 and 93 respectively in. the form of diode gates for interstage isolation purposes. The outputs from 89, 91 and 93 are inverted by slow speed switching and inversion means 95, 97 and 99, respectively. Outputs 101, 103 and 105, respectively, are sent to a plurality of AND gates 107, 109, 111, 113, 115 and 117 in accordance with an algebraic simplification of Table 1. Outputs 101, 103 and 105 are also routed to signal translating means 119, 121 and 123, respectively, and inverted and delayed by slow speed switching and inversion means 125, 127 and 129. Outputs 131, 133 and 135 are routed in accordance with an algebraic simplification of Table 1 to AND gates 107 through 117.

Coincidence of three binary ls as inputs to any of the first group of AZND gates 107, 109 or 111 results in a signal being sent to solenoid driver unit 19-1 which energizes the right roller assembly 25, so that its associated rollers are no longer pulling on the ribbon which causes the ribbon to skew to the left under the influence of left roller assembly 23 until it returns to its home position. Conversely, coincidence of three binary ls as inputs to any of the second group of AND gates 113, 115 or 117 results in a signal being sent to solenoid driver unit 19-2 which energizes the left roller assembly 23, so that its associated rollers are no longer pulling on the ribbon which causes the ribbon to skew to the right under the influence of right roller assembly 25 until it returns to its home position. In either case, when the ribbon returns to its home position, the affected roller is de-energized due to the loss of signal coincidence at the qualified gate.

Under certain tracking-conditions such as when the ribbon is too-wide, individual AND gates in both the first and second groups are simultaneously qualified, resulting in signals being sent to both solenoid driver units 191 and 19-2. In this situation, both the right and left solenoid roller assemblies 23 and 25, respectively, are energized, resulting in no rollers pulling.

The purposes for the slow speed switching and inversion means and the second translation and inversion of each signal from the photo-electric sensing units are to provide positive levels of energy to drive the AND gates for both the true and false" levels of A, B and C. That is, the first set of inverters provide K, 1 and 6, while the second set of inverters provide A, B and C levels for the gating requirements.

FIG. 9 is a schematic representation of one type of amplifier driver which may be satisfactorily used in the solenoid driver unit 19.

Applicants have provided a relatively simple and inexpensive web tracking, indicating and correcting device which is sensitive to a plurality of tracking conditions. Although the description has referred to relatively Wide ribbons for high speed printing application, it is obvious that applicants invention may be utilized with ribbons of most any width possible traveling at most any linear ribbon velocity and printing speeds and in various other fields such as paper manufacture or steel sheet fabrication. It should be noted that the scope of this invention includes various changes and modifications that are within the skill of those familiar with the art, the scope of the invention being limited only by the following claims.

We claim:

1. In a machine for moving a Web of indeterminate length past a processing station, a tracking device comprising:

first photo-electric sensing means, having at least two photo-electric elements and a first light source mounted for cooperation with one longitudinal edge of said web, at least one of said photo-electric elements normally being blocked and at least one other of said photo-electric elements being normally exposed with respect to said light source,

second photo-electric sensing means, having at least one photo-electric element and a second light source, mounted for cooperation with the other longitudinal edge of said Web, said photo-electric element of said second photo-electric sensing means normally being blocked with respect to said second light source, said first and second photo-electric sensing means having binary outputs,

logic means for gating preselected combinations of said binary outputs, said gated combinations corresponding respectively to a plurality of different conditions of said web,

a pair of electromagnetically adjustable roller assemblies mounted for selectively exerting rolling pressure on opposite longitudinal edges of said web, and

driver means for actuating said pair of electromagnetically adjustable roller assemblies, either individually or in combination, in response to selected gated combinations from said logic means.

2. The device in claim 1 including additional means for controlling said machine and for indicating said diiferent web conditions in response to said gated combinations from said logic means.

3. The device in claim 2 wherein said indicating means includes additional alarm means for signaling a Web malfunction.

4. In a machine having means for controlling movement of a web of indeterminate length past a processing station, a web condition detecting device comprising:

a photo-electric sensing unit having first and second photo-electric elements and a common light source mounted for detecting two directions of transverse movements of one longitudinal edge of said Web, one of said elements being blocked and the other of. said photo-electric elements being exposed with respect to said light source by said web when said web is in its home position,

another photoelectric sensing unit having a third photoelectric element and an associated light source mounted for detecting transverse movement in one direction of the other longitudinal edge of said web, said third photo-electric sensing element being blocked with respect to its light source by said Web when said Web is in said home position, each of said three photo-electric sensing elements having a binary output, and

logic means for signaling said web control means in response to preselected combinations of said binary outputs, said binary combinations being representative of at least three different conditions of normal and abnormal web movement with respect to said home position.

5. In a machine for moving a web of indeterminate length past a processing station, a device for indicating and correcting different web tracking conditions compriselectronic detecting means responsive to the tracking position of said moving web for generating a unique three-digit binary signal representing each of N normal and abnormal tracking conditions of said moving Web, wherein N is an integer greater than 2,

logic means for gating preselected combinations of said three binary signals, means engageable with said web and responsive to said gated signals for laterally shitting said web to correct selected abnormal web conditions, and

means responsive to said gated signals for selectively indicating the normal and abnormal web conditions detected.

6. In a machine for moving a Web of indeterminate length past a processing station, a tracking device comprising:

a first photo-electric sensing element positioned for detecting and signaling the presence and absence of one longitudinal edge of said web,

a second photo-electric sensing element positioned proximate said first element along an axis substantially perpendicular to the motion of said web, said second element being positioned for detecting and signaling the presence and absence of said one longitudinal edge of said web,

a third photo-electric sensing element positioned for detecting and signaling the presence and absence of the other longitudinal edge of said web,

a first light source common to and positioned proximate said first and second photo-electric sensing elements,

a second light source positioned proximate said third photo-electric'sensing element,

a plurality of three input AND gates,

first means for individually inverting and delaying the signals from said three photo-electric sensing elements,

first means for individually applying said inverted and delayed signals to said plurality of AND gates in a predetermined pattern,

second means for individually inverting and delaying the signals from said first inverting and delaying means,

second means for individually applying said second invetted and delayed signals to said plurality of AND gates in a predetermined pattern,

a first solenoid-controlled, roller assembly for selectively exerting rolling, pinching contact with one edge of said web,

first solenoid driver means connected to the outputs of preselected ones of said plurality of AND gates for energizing said first solenoid roller assembly in response to signals from said selected AND gates,

a second solenoid controlled, roller assembly for selectively exerting rolling, pinching contact with the other edge of said web, and

second solenoid driver means connected to the outputs of preselected ones of said plurality of AND gates for energizing said second solenoid roller assembly in response to signals from said selected AND gates.

7. In a high speed printer having a direction reversible wide ink ribbon, a ribbon tracking and correcting device comprising:

first photo-electric sensing means, having at least two photo-electric elements mounted for sensing and binary signaling the presence and absence of one longitudinal edge of said ribbon,

second photo-electric sensing means, having at least one photo-electric element mounted for sensing and binary signaling the presence and absence of the other longitudinal edge of said ribbon,

logic means for gating preselected combinations of said binary output signals, said combinations being representative of a plurality of difierent web tracking conditions,

a pair of electromagnetically adjustable, roller assemblies pivotally mounted for pressure cooperation with opposite longitudinal edges of said ribbon,

means for selectively energizing said roller assemblies in response to selected gated combinations from said logic means, and

alarm means for signaling ribbon tracking malfunctions in response to selected gated combinations from said logic means.

8. In a machine for moving a web of indeterminate length past a processing station, a device for detecting, correcting and indicating web tracking conditions comprising:

first sensing means including at least two sensing ele-' element having a binary output and being mounted.

for cooperation with the other longitudinal edge of said web, said sensing element of said second sensing means being normally energized by the presence of said web,

gating means for signaling coincidence of preselected combinations of said binary outputs, said combinations corresponding respectively to a plurality of dif- I ferent web conditions,

means responsive to said gated signals for correcting selected abnormal web conditions, and

means responsive to said gated signals for selectively indicating the web conditions detected.

References Cited UNITED STATES PATENTS 2,842,361 7/ 1958 Miller 22621 3,086,121 4/ 1963 Cockrell 250209 3,147,898 9/1964 Huck 22617 3,153,721 10/1964 Kaminski et al. 250219 3,246,202 4/1966 Rhodes 250-208 STANLEY H. TOLLBERG, Primary Examiner. 

